As the process for producing an acid anhydride, there has been a process which comprises reacting a low-molecular acid anhydride such as acetic anhydride with a high-molecular carboxylic acid [for example, J. Am. Chem. Soc. 63 699 (1941)], or a process which comprises reacting a low-molecular acid anhydride with a high-molecular carboxylic acid chloride to form a high-molecular acid anhydride (for example, JP-A-5-339194). In these processes, the low-molecular carboxylic acid formed as a by-product or a carboxylic acid chloride is removed by azeotropic distillation with a solvent. With these processes, however, it is difficult to produce a low-molecular acid anhydride and the yield thereof is low. Further, this process has a problem in that it is unapplicable to production of an acid anhydride which is thermally unstable.
As the process for producing a low-molecular acid anhydride, there is known a process which comprises reacting an acid halide with a carboxylic acid and wherein the inorganic acid formed as a by-product is removed (for example, JP-A-9-104642) or a process which comprises reacting an acid halide with a metal carboxylate and wherein the metal salt formed is removed [for example, J. Org. Chem. 60(7) 2271 (1995)]. Even with these processes, however, the yield of the acid anhydride obtained is not satisfactory. Further, the reaction using an acid halide is inferior in the safety of reaction.
Meanwhile, as the process for producing a low-molecular acid anhydride at a good yield, there is known a process which comprises reacting a carboxylic acid with methanesulfonyl chloride in the presence of triethylamine [J. Chem. Res., Synop. 3 100 (1984)]. In this process, 2 equivalents of a carboxylic acid is reacted using 2.1 equivalents of methanesulfonyl chloride [therefore, the acids (hydrochloric acid and methanesulfonic acid) formed by the reaction with methanesulfonyl chloride become 2.1 equivalents] and 3.33 equivalents of triethylamine (this trimethylamine amount becomes 1.67 equivalents relative to the acids formed from methanesulfonyl chloride). In this reaction, triethylamine is used in an amount fairly larger than the stoichiometric amount required for the raw material carboxylic acid and methanesulfonyl chloride in order to accelerate the reaction. This reaction is conducted at −15° C. using tetrahydrofuran as a solvent.
After the reaction, tetrahydrofuran is distilled off under reduced pressure; ethyl acetate is added to the residue for dissolution; and the ethyl acetate phase is washed with an aqueous sodium hydrogencarbonate solution. Thereafter, the ethyl acetate phase is dried over anhydrous sodium sulfate, after which the solvent therein is distilled off to obtain an intended acid anhydride.
The above reaction is highly reactive and tends to give rise to a side reaction. As a result, the yield tends to be inferior and the reaction has a room for improvement. Particularly when the carboxylic acid is a polymerizable group-containing carboxylic acid such as acrylic acid or methacrylic acid [hereinafter the both are referred to as (meth)acrylic acid], a fairly large proportion of the acid anhydrides obtained is a polymer(s), which greatly reduces the yield of an intended product.
Further in the above process using a large amount of triethylamine, washing with water is conducted after the reaction in order to remove triethylamine from the product; however, the compound has a strong odor and is unable to remove sufficiently.
Furthermore, a triethylamine salt is contained in a large amount in the waste solution generated from the above washing with water, which poses a problem of waste solution disposal.
Thus, it is desired to develop a process for producing an acid anhydride, which can produce an acid anhydride at a high yield and a high purity, wherein the purification of intended product from reaction mixture is easy, and wherein the waste solution disposal is easy as well.